1. Field of the Invention
The present invention relates to improvements in thermoformed plastic containers for food or other articles and particularly to latching structure for holding the lid and base of the container in closed condition.
2. Description of Prior Art
Containers for food or other articles thermoformed from plastic material normally comprise a lid and base which may be hinged to each other or may be separate parts. In order to hold the lid and base of the container in closed condition, it is necessary that the lid and base have latching structure which can be manually opened and closed with relative ease. However, the structure should be capable of locking the container in closed condition to avoid accidental opening.
Various types of latching or locking structures have been proposed in the past. For example, cylindrical male and female locking elements have been proposed as disclosed in U.S. Pat. No. 4,576,330. Elongated rib structures comprising cooperating male and female elements have been proposed as disclosed in British Patent Specification 1,418,897 and in Canadian Patent 1,117,491. In the prior elongated rib structure locking elements the locking action has taken place on the long sides of the ribs.
Mating male and female rib elements where the locking takes place at the shorter end or opposed sides of the ribs, rather than along the longer sides of the ribs is disclosed in U.S. Pat. No. 5,046,659. It is claimed that this permits the latching structure to be located at a variety of locations on the container and provides for latching action while permitting ease of manual operation in opening and closing the latching structure on the container.
However, the rectangular nature of the cross-section of the male and female locking elements still makes it difficult to repeatedly open and close the latch, although locking takes place along opposite sides or ends only, since the close proximity of the unlatched sides when the latch is engaged causes adhesion by friction or even an interference fit when close tolerances are not observed during production.
Further, the sharp and well defined locking elements or ribs which interengage are difficult, if not impossible, to manufacture in a thermoforming process, substantially increasing the cost of production.
If one could form latches as distinct as what is disclosed in the '659 patent, subsequent stripping of the formed part, out of the mold, would be difficult if not impossible. The mechanism would be sophisticated and more expensive than what is presently in use for part stripping, as any sharp well-defined corners in the mold and formed latch would tend to adhere without some type of mechanical separation. This would result in a more expensive production tool. Further, depending on the exact mechanism designed to accomplish the stripping, the stripping action could adversely affect the cycle time (i.e., increase it). Forming the well defined, distinct shoulders disclosed in the '659 patent would likely require a longer cycle time than what is presently in use. The plastic would need to be heated more, and the pressure form dwell would likely need to increase. Again, all these factors lead to increased production costs. Cycle time increases would result in more machine time (i.e., station) and labor expense for each container. As indicated, another issue is whether or not the '659 structure can even be manufactured, i.e., whether a mold could even be configured to provide the resultant product having sharply defined corners and undercuts. The fluid material would tend to flow around the sharp corners of the mold, forming arcuate surfaces.